The rental car companies have vans that meet the ships arriving on Kauai. It's about a 5-minute ride to the Lihue Airport where you pick up the rental cars.
We've pre-reserved Alamo on each of the islands in advance for our 4/17/01 Infinity cruise. From what I've read here, that seems to be the best way to go. We have no intentions of booking shore excursions on this cruise. I'm sure rental cars will suit us just fine.